1. Field of the Invention
The present invention relates to a magnetic random access memory (MRAM) in which a MTJ (Magnetic Tunnel Junction) element for storing “1”, “0”-data by a tunneling magneto resistive effect is used to constitute a memory cell.
2. Description of the Related Art
In recent years, a large number of memories in which data is stored by a new principle have been proposed, and among the memories, there is a memory which has been proposed by Roy Scheuerlein et. al. and in which a tunneling magneto resistive (hereinafter referred to as TMR) effect is used (e.g., see ISSCC2000 Technical Digest p.128 “A 10 ns Read and Write Non-Volatile Memory Array Using a Magnetic Tunnel Junction and FET Switch in each Cell”).
In the magnetic random access memory, “1”, “0”-data is stored by a MTJ element. As shown in FIG. 1, the MTJ element includes a structure in which an insulating layer (tunnel barrier) is held between two magnetic layers (ferromagnetic layers). The data stored in the MTJ element is judged by judging whether directions of spins of two magnetic layers are parallel or anti-parallel to each other.
Here, as shown in FIG. 2, “parallel” means that the directions of spins of two magnetic layers (magnetization directions) are the same, and “anti-parallel” means that the directions of spins of two magnetic layers are opposite (the directions of arrows indicate the directions of spins).
It is to be noted that an anti-ferromagnetic layer is usually disposed in one of two magnetic layers. The anti-ferromagnetic layer is a member for fixing the direction of spins of the magnetic layer on one side and changing only the direction of spins on the other side to easily rewrite data.
The magnetic layer whose direction of spins is fixed is referred to as a fixed or pinned layer. Moreover, the magnetic layer whose direction of spins can freely be changed in accordance with write data is referred to as a free or storage layer.
As shown in FIG. 2, when the directions of spins of two magnetic layers are parallel to each other, tunnel resistance of the insulating layer (tunnel barrier) held between these two magnetic layers becomes lowest. This state is a “1”-state. Moreover, when the directions of spins of two magnetic layers are anti-parallel to each other, the tunnel resistance of the insulating layer (tunnel barrier) held between these two magnetic layers becomes highest. This state is a “0”-state.
Next, a write operation principle with respect to the MTJ element will briefly be described with reference to FIG. 3.
The MTJ element is disposed in an intersection of a write word line and data selection line (read/write bit line) which intersect with each other. Moreover, write is achieved by passing a current through the write word line and data selection line, and using a magnetic field made by the current flowing through opposite wirings to set the direction of spins of the MTJ element to be parallel or anti-parallel.
For example, a magnetization easy axis of the MTJ element corresponds to an X direction, the write word line extends in the X direction, and the data selection line extends in a Y direction crossing at right angles to the X direction. In this case, at a write time, the current flowing in one direction is passed through the write word line, and the current flowing in one or the other direction is passed through the data selection line in accordance with write data.
When the current flowing in one direction is passed through the data selection line, the direction of spins of the MTJ element becomes parallel (“1”-state). On the other hand, when the current flowing in the other direction is passed through the data selection line, the direction of spins of the MTJ element becomes anti-parallel (“0”-state).
A mechanism in which the direction of spins of the MTJ element changes is as follows.
As shown by a TMR curve of FIG. 4, when a magnetic field Hx is applied in a long-side (easy-axis) direction of the MTJ element, a resistance value of the MTJ element changes, for example, by about 17%. This change ratio, that is, a ratio of a resistance value before the change to that after the change is referred to as an MR ratio.
It is to be noted that the MR ratio changes by a property of the magnetic layer. At present, the MTJ element whose MR ratio is about 50% has also been obtained.
A synthesized magnetic field of the magnetic field Hx of the easy-axis direction and magnetic field Hy of a hard-axis direction is applied to the MTJ element. As shown by a solid line of FIG. 5, a size of the magnetic field Hx of the easy-axis direction necessary for changing the resistance value of the MTJ element also changes by the size of the magnetic field Hy of the hard-axis direction. This phenomenon can be used to write the data into only the MTJ element existing in the intersection of the selected write word line and data selection line among arrayed memory cells.
This state will further be described with reference to an asteroid curve of FIG. 5.
The asteroid curve of the MTJ element is shown, for example, by the solid line of FIG. 5. That is, when the size of the synthesized magnetic field of the magnetic field Hx of the easy-axis direction and magnetic field Hy of the hard-axis direction is outside the asteroid curve (solid line) (e.g., positions of black circles), the direction of spins of the magnetic layer can be reversed.
Conversely, when the size of the synthesized magnetic field of the magnetic field Hx of the easy-axis direction and magnetic field Hy of the hard-axis direction is inside the asteroid curve (solid line) (e.g., positions of white circles), the direction of spins of the magnetic layer cannot be reversed.
Therefore, when the sizes of the magnetic field Hx of the easy-axis direction and magnetic field Hy of the hard-axis direction are changed, and the position of the size of the synthesized magnetic field in an Hx-Hy plane is changed, the write of the data with respect to the MTJ element can be controlled.
It is to be noted that read can easily be performed by passing the current through the selected MTJ element, and detecting the resistance value of the MTJ element.
For example, switch elements are connected in series to the MTJ elements, and only the switch element connected to a selected read word line is turned on to form a current path. As a result, since the current flows only through the selected MTJ element, the data of the MTJ element can be read.
In a magnetic random access memory, as described above, the data is written by passing a write current through the write word line and data selection line (read/write bit line), and allowing the synthesized magnetic field generated thereby to act on the MTJ element.
Therefore, in order to write the data with good efficiency, it is important to apply the synthesized magnetic field to the MTJ element with good efficiency. When the synthesized magnetic field is efficiently applied to the MTJ element, reliability of the write operation is enhanced, the write current is further reduced, and low power consumption can be realized.
Then, in recent years, a magnetic random access memory has been proposed including a structure in which a part of the surface of the write word line and data selection line is coated with a yoke material having an effect of concentrating a magnetic flux and having a high permeability.
In order to write the data efficiently and exactly, as described above, it is important to apply the synthesized magnetic field to the MTJ element with good efficiency. It is also important to impart the same characteristic (TMR curve) to MTJ elements which constitute the memory cell array for the following reason. When the characteristics of the MTJ elements differ, and even when the data is written on the same conditions, the MTJ element in which exact data cannot be written is generated with an increasing possibility.
However, in actual, because of various problems in a manufacturing process (flatness of the substrate of the memory cell, dispersion of a film thickness in a wafer, and the like), the characteristics (TMR curves) of the MTJ elements are not the same. It is to be noted that as direct causes for the dispersion of the characteristics, a leak magnetic field (magnetostatic coupling) which acts on the free layer (storage layer) from the pinned layer (fixed layer) of the MTJ element, and a phenomenon (topological coupling) in which a tunnel barrier layer of the MTJ element is not flat and has a pole are assumed.
As a result, for example, when ideal characteristics are shown by the TMR curve in FIG. 4, the MTJ elements having the characteristics shown in FIGS. 6 and 7 coexist in the wafer or chip. In this case, for example, in design, even when the write current sufficient for changing the magnetization direction (direction of spins) of the MTJ element is passed, the magnetization direction of the MTJ element does not change. This situation occurs, and the reliability of the magnetic random access memory drops.
Therefore, in the magnetic random access memory, when the dispersion of the characteristic (TMR curve) of the MTJ element can be corrected after assembly, this can contribute to the enhancement of the reliability.